1. Field of the Invention:
The present invention relates generally to digital techniques for processing analog signals; it relates more particularly to receiver for a pulsed Doppler radar system.
2. Description of the Prior Art:
The basic structures and operation of Doppler radar systems are described in different technical publications. For example, the work by M. I. SKOLNIK called "Radar Handbook", 1970, published by Mc Graw-Hill describes such basic structures. Referring more particularly to the chapters relating to moving target visualization techniques (VCM) using digital techniques for processing the echo signals supplied by the receiver, these techniques are known under the name DMTI (Digital Moving Target Indicator).
In known VCM radar receivers, the output of the selective intermediate frequency amplifier (IF) is connected to two phase detectors which operate in phase quadrature to supply bipolar video frequency signals representing the in phase (I) and in quadrature (Q) components of the output signals of the IF amplifier. The output of each of these phase detectors is connected to a low-phase filter, the output of which is sampled by a sampler-disabler controlled by a clock signal synchronous with the signal triggering off the transmission pulses. The frequency of this clock signal is substantially equal to twice the inverse of the pulse duration (.delta.). The amplitude of the output samples of the sampler-disabler is coded on P.sub.o levels by an analog-digital coder for supplying words of N.sub.o binary digits such that P.sub.o =S.sup.No. Subsequently, a digital processor carries out, in real time, on the digital signals output by each of these two A/D coders), filtering operations intended to discriminate the echos of the moving targets and to reject the echos of fixed targets and the ground return.
In the VCM radar receivers of the prior art, the particularly critical elements are the sampler-disablers and the A/D coders, which elements may operate over the whole amplitude dynamic of the echo signals. In practice, A/D coders are formed capable of providing a high number P.sub.o of levels, for example 4096 levels corresponding to words of 12 binary digits, including the sign bit, and this at rates of the order of 1 MHz or more. These devices for digitally coding the instantaneous amplitude of the output signals of the phase detectors are of delicate construction and their manufacturing cost remains extremely high. Another disadvantage of these radar receivers of the prior art is related to the fact that the filter "matched" to the pulse signals transmitted by the radar transmitter is formed for intermediate frequency operation (IF); however, it has already been proposed to form the "matched" filter for video frequency operation each of the signal channels I and Q. However, in all cases, the matched filters as presently formed are an approximation of the "optimum matched" filter, which is a first cause of degradation in the detection performance of the radar system. A further disadvantage of these conventional receivers comes from the fact that technologically the sampling rate of the output signals from the phase detectors remains limited if the amplitude resolution performance cannot be sacrificed (number of levels P.sub.o). In addition, even if the NYQUIST-SHANON sampling criterion is complied with, that is to say that the sampling rate of the output signals of the phase detectors should be substantially equal to twice the inverse of the duration of the pulse signals transmitted, the echo signals are not always sampled at the precise moment when their level is maximum. The result is a second cause of degradation of the optimum detection performance of the radar system.
The aim of the invention is to overcome the above mentioned drawbacks of the radar receivers of the prior art.